Constant current electrical circuits



Feb. 20, 1945. L. w. HOUGHTON CONSTANT CURRENT ELECTRICAL CIRCUIT FiledJan. 6. 1943 Inventor: Anna w,

Attorney Patented Feb. 20, 1945 CONSTANT CURRENT ELECTRICAL CIRCUITSLeslie Wilfred Houghton, London, England Application January 6, 1943,Serial No. 471,395 In Great Britain May 29, 1942 6 Claims.

The present invention relates to arrangements for stabilising thecurrent in electrical circuits, and in particular employs thermallysensitive devices known as thermistors.

Thermistors have been in use for some years and are characterized by atemperature coeflicient of resistance which may be either positive ornegative and which is moreover many times the corresponding coefllcientfor a pure metal such as copper. This property renders thermistorsparticularly suitable for a variety of special applications in electriccircuits.

Various different materials are available for the resistance element ofa thermistor, these various materials having different properties inother respects; as one example, a resistance material having a highnegative temperature coeflicient of resistance comprises a mixture ofmanganese oxide and nickel oxide, with or without the addition ofcertain other metallic oxides, the mixture being suitably heat treated.

Thermistors have been employed in two different forms: (a) known as 9.directly heated thermistor and comprising a resistance element of thethermally sensitive resistance material provided with suitable lead-outconductors or terminals, and (b) known as an indirectly heatedthermistor comprising the element (a) provided in addition with aheating coil electrically insulated from the element. A directly heatedthermistor is primarily intended to be controlled by the current whichflows through it and which varies'the temperature and also theresistance accordingly. Such a thermistor will also be affected by thetemperature of its surroundings and may therefore be used forthermostatic control and like purposes with or without direct heating bythe current flowing through it. An indirectly heated thermistor ischiefly designed to be heated by a controlling current which flowsthrough the heating coil and which will usually, but not necessarily, bediflerent from the current which flows through the resistance element,but this typ of thermistor may also be subjected to either or both ofthe types of control applicable to a directly heated thermistor.

More detailed information on the properties or thermistors will be foundin an article by G. L. Pearson in the Bell Laboratories Record, December1940, page 106.

The present invention employs two indirectly heated thermistors for thepurpose of supplying a constant current to a load which may be variablefrom a source 0! voltage which may also be variable.

According to the invention, there is provided a circuit arrangement forsupplying a constant current to a load from a source of voltage, eitheror both of which may be variable, comprising a first indirectly heatedthermistor and a second indirectly heated thermistor, the resistanceelement of the first thermistor being connected to the variablesource inseries with the heating coil of the second thermistor, the heating coilof the first thermistor and the resistance element of the secondthermistor being supplied with current from a local source ofelectromotive force.

The invention will be more clearly understood by reference to thedetailed description which follows, and to the accompanying drawingwhich shows schematic circuit diagrams of arrangements according to theinvention.

In the drawing:

Fig. 1 is a schematic circuit diagram illustrating the invention in apreferred form.

Fig. 2 is a modification thereof.

The diagram of Fig. 1 shows two indirectly heated thermistors T1 and T2,which have a negative temperature coeflicient of resistance. Theresistance element R1 of T1 is connected in series with the heating coil1': of T2 and with a load L to two terminals I and 2, across which thereexists a difference of potential V which may be continuous oralternating. The heating coil 1'1 of T1 and the resistance element R2 ofT2 are connected in parallel to a local direct current or alternatingcurrent source S having an adjustable constant resistance R: connectedin series therewith. The source 8 should preferably supply asubstantially constant voltage.

It is assumed that the resistances T1 and T2 of the heating coils areapproximately constant. Suppose that it is required to produce aconstant current I in the load L irrespective of the variations of V orof the load L. The resistance R1 will depend partly on the current I andpartly on the current 11 flowing through 1-1. The latter depends on R3and R2 (assuming the source S has a constant voltage). Now suppose thatthe voltage V increases, or the load resistance decreases, or both. Thecurrent I will momentarily increase, and this will raise the temperatureof T2 which reduces R2. This will in turn reduce the current I1 through11 and the temperature of T1 raising the value of the resistance R1. Theincrease in I will also tend to raise the temperature of T1 thuslowering R1. By adjusting Ra it is possible to arrange so that the neteffect is to lower the temperature of T1 thus raising its resistance bythe amount required to compensate for the change in V and/or L, so thatthe current I is brought back to the original value.

Similarly, when the voltage V decreases, or the load resistanceincreases, or both, the reverse process takes place, and the momentarydecrease of I is very quickly corrected. To obtain the best results, R2should be smaller than n and Rs should be large compared with R2.

If thermistors having positive temperature coefficients are'employed,the circuit shown in the figure will be seen to operate also in thedesired manner. For when the current I tends to increase, thetemperature of T2 is raised as before, but Re now increases therebyincreasing the current II. This raises the temperature of T1 whichincreases R1 as desired, to restor I to its original value.

If one thermistor has a positive temperature coeflicient and one has anegative temperature coefiicient the desired operation is obtained by asimple modification of the circuit as shown in Fig. 2. In thismodification, n and R2 are connected in series with R3 and S instead ofin parallel. If T1, for example, has a negative temperature coeflicient,it is clear that if I should increase, the rise in temperature of T:will cause T1 to cool so that the resistance R1 increases as desired. IfT2 has the negative coefficient the reverse process occurs, againincreasing R1. Adjustment of R3 enables the necessary amount ofcompensation to be obtained in either case.

By suitable design of the thermistors it can be arranged so that theheating effect of'th current which passes through the resistance elementis negligible, in which case only the efiect produced by the heatingcoils need be taken into account. It is to be noted however that when T1has a positive temperature coeflicient, the heating by the current Iwill aid the desired compensating effect, while when it has a negativetemperature coefii'cient, as already explained, it will oppose thedesired effect.

aseao'ro stant current to a load from a source of voltage,

either or both of which may be variable, comprising a first indirectlyheated thermistor and'a second indirectly heated thermistor, theresistance element of the first thermistor being connected to thevariable source in series with the heating coil of the secondthermistor, the heating coil of the first thermistor and the resistanceelement of the second thermistor being supplied with current from alocal source of electromotive force.

2. A circuit arrangement according to claim 1 in which both thermistorshave a temperature coeflicient of resistance of the same sign, theheating coil of the first thermistor and the resistance element of thesecond thermistor being connected in parallel.

3. A circuit arrangement according to claim 1 in which the thermistorshave temperature coefficients of resistance of opposite sign, theheating coil of the first thermistor and the resistance element of thesecond thermistor being connected in series.

4. A circuit arrangement according to claim 1 in which the thermistorsboth have negative temperature coefficients of resistance.

5. A circuit arrangement according to claim 1 in which the firstthermistor has a positive temperature cofficient of resistance.

6. A circuit arrangement according to claim 1 in which an adjustableconstant resistance is connected in series with the local source ofelectromotive force.

LESLIE WILFRED HOUGHTON.

